In directional drilling practice, the course of a wellbore is determined by measuring downhole the direction of the wellbore with inclinometers and magnetometers at discrete survey points, mostly taken after drilling one stand of pipe and making a connection. The directional sensors provide an inclination angle α with respect to vertical and an azimuth angle β with respect to magnetic North. Complemented with the measured depth at the survey points, a series of survey stations may be obtained. In most directional applications, the minimum curvature method is applied to calculate the course of the wellbore from the survey stations. The minimum curvature method assumes a circular arc between the survey stations with constant curvature or constant dogleg severity (DLS). Additional effects such as local curvature variations can generate significant depth errors, in particular with steerable motor systems if operated alternately in sliding and rotating mode between the survey stations. In addition, standard directional sensors can be affected by magnetic interference, either from near-by wells or in casing exit operations.